The present invention relates generally to gas turbine engines, and, more specifically, to an oil cooling system therein.
A typical gas turbine engine used for powering an aircraft in flight includes a fan having one or more stages followed in turn by a multi-stage high pressure compressor which provides compressed air to a combustor wherein it is mixed with fuel and ignited for generating combustion gases which flow downstream to a high pressure turbine (HPT) and in turn to a low pressure or power turbine (LPT), which turbines power the high pressure compressor and fan, respectively. A rotor shaft joins the fan to the LPT and is suitably mounted in bearings, and another rotor shaft joins the compressor to the HPT with additional bearings being used.
In one exemplary turbine engine, five main bearings are axially spaced apart along the engine from the fan to the LPT, and three axially spaced apart oil sumps are provided for channeling lubricating oil to the several bearings as well as to a conventional accessory gearbox. In this example, the number one bearing associated with the fan is lubricated from a forward sump, the number two and three bearings and the accessory gear box are lubricated from the intermediate sump, and the number 4 and five bearings are lubricated from an aft sump.
The several sumps are typically connected to a common oil tank which in turn is connected to an oil filter and then to a counterflow, two circuit heat exchanger or oil cooler. A conventional multi-element pump pumps the oil from the several sumps to the oil tank and through the oil filter to the heat exchanger. The primary, or cooling circuit of the heat exchanger is conventionally joined to a fuel tank in the aircraft which provides a heat sink in the relatively cool fuel which is used in the heat exchanger for cooling the relatively hot oil channeled thereto from the sumps. The oil cooled in the heat exchanger is typically returned to all three sumps for providing filtered and cooled oil at one temperature as provided by the single oil outlet of the heat exchanger.
In one turbine engine being developed, the number 4 bearing is a relatively highly loaded differential bearing mounted between the two rotors of the HPT and the LPT whose life is sensitive to the temperature of the cooling oil provided thereto. It is desirable to provide cooled oil to the number 4 differential bearing at a lower temperature than that oil supplied to the other bearings for improving the useful life thereof.